Buckling wire or beam probes have come into vogue for facilitating the testing of electrical characteristics of integrated circuits connected to pads on a semiconductor chip wherein the flexing or buckling wire probes engage at opposite ends, the pads and contact points on the tester space transformer die by relative movement of the substrate toward the space transformer die. A typical buckling beam probe assembly is set forth in U.S. Pat. No. 3,806,801 issued Apr. 23, 1974, to Ronald Bove and assigned to the common corporate assignee.
The application of axial forces on the wire probes causes the wire probes or beams to buckle laterally under axial compressive force, the result of which is to insure low ohmic contact of each current conducting wire probe, at respective ends, to the space transformer die contact and the chip carried pad. If the force of the probe engaging the pad exceeds that for which the pad or chip has been designed, then the pad and/or chip may be damaged. Such systems are complicated by the fact that it is necessary to space the probes sufficiently from each other to enable such deflection without the probes contacting each other and shorting out the probes.
Referring to FIG. 1 of the drawings, in a typical buckling beam probe assembly 10, the underlying substrate 18 is placed some distance from the overlying space transformer die 12 carrying the potted wire contacts 14 whose ends 16 are exposed at the bottom of the space transformer die 12. Typically, a top mating locating guide 22 and a bottom locating guide 34 are provided in juxtaposition to the space transformer die 12 and a substrate 18, respectively, but spaced somewhat therefrom, and wherein both of these locating guides 22, 34 are provided with small diameter holes 24 and 36, respectively, sized slightly larger than the diameter of the probes passing therethrough, so as to slide therein. Thus the wire probes 38, absent deflection, are aligned throughout their axes with the wire contacts 14 of the space transformer die 12, and pads 20 on the substrate 18. In addition, there is normally provided an offset die 26 whose holes 28 through which the wire probes 38 pass, are offset relative to the holes 24 of the locating guides to thus bias the wire probes to buckle laterally, in a given direction. Additionally, a floating die 30 is employed intermediate of the offset die and the lower locating guide to isolate and insulate the probe wires such that during axial applied force, the wires are axially deflected, and under the applied axial force 40, the lateral deflections result in a controlled manner and in preset directions defined by the offset die. Typically, all guides and dies are connected to the space transformer die 12 and movable as an assembly relatively toward and away from the underlying substrate 18.
FIGS. 2 and 3 are plots which show applied force versus axial deflection, and lateral deflection versus applied axial deflection for such typical buckling beam probe assemblies. In FIG. 2, under initial applied axial force, the wire probes deflect axially to a limited degree during initial lateral deflection and then to a greater degree when the axial force reaches a predetermined value. In FIG. 3, the lateral versus axial deflection curve shows that, for small axial deflections, that there are relatively large lateral deflections. This characteristic limits the application of buckling beam probes for two main reasons. First, when probing a complex (dense) pattern, only very small probe axial deflections can be handled. The wire probes tend to come into lateral contact with each other and short out. Even if floating die is used to isolate the probes and to thus insulate them electrically, it too is limited due to the floating die causing some of the probes to buckle without achieving contact. Secondly, even if the lateral deflections are not critical for contact reasons, a large buckle will be constricted due to the increase in frictional force of the dies (binding effect on the wire probes). In addition, too much axial deflection will cause the beams, i.e. the probe wires, to permanently buckle. Therefore, there are substantial limits in the use of buckling beam probes, as conventionally fabricated, since complex (dense) patterns need very uniform probing surfaces and the overall axial deflections of conventional buckling beams are limited to about five mils maximum.
Some attempts have been made to provide buckling beam probes whose probe wires operate under multiple mode buckling. Such concepts are set forth in IBM Technical Disclosure Bulletin, Vol. 17, No. 2, July 19, 1974, pages 444 and 638. In both these disclosures, the multiple buckling modes of the connectors/contactors are independent, i.e. separate, which while being a step in the right direction, require isolation of the wire probes into two halves, one to the connector side and the other to the contactor side.
It is, therefore, a primary object of the present invention to provide a buckling beam probe assembly which permits increase in the axial deflections of the wire probes while maintaining desired force deflection characteristics, and in which lateral deflections are decreased, permitting an increase in density of the probing pattern and allowing the probing of more varied probing surfaces, and wherein the buckling beam assembly may be tailored to a user's specific needs by use of multiple mode buckling.